If a lady is fasting and her husband stimulates her with his fingers till she reaches climax,
does ghusl become wajib on her?
does her fast become void?

In this case since you have not consummated ghusl will not be wajib on her and her fast remains valid. However such act has to be avoided at all time during fast it may lead to unwanted act in during fast or even take one out of the mood of fasting